Traceability for Microwave

Remote-Sensing Radiometry

J. Randa

NISTIR 6631
2

NISTIR 6631

Traceability for Microwave

Remote-Sensing Radiometry

J. Randa

Electromagnetics Division

325 Broadway

Boulder, CO 80305

June 2004

U.S. Department of Commerce

Donald L. Evans, Secretary

Technology Administration

Phillip J. Bond, Under Secretary for Technology

National Institute of Standards and Technology

Arden L. Bement, Jr., Director
3

TRACEABILITY FOR MICROWAVE REMOTE-SENSING

RADIOMETRY

J. Randa

Electromagnetics Division

National Institute of Standards and Technology

Boulder, CO 80305

Abstract: A plan is presented for developing a standard for brightness temperature at

microwave frequencies, as well as two different methods for transferring this standard to

the microwave remote-sensing community. The standard will be based on existing NIST

waveguide and coaxial radiometers, which are calibrated with primary noise standards.

The radiometers will be converted (reversibly) to standard remote-sensing radiometers by

connecting characterized antennas to the plane at which the device under test is

connected in normal use. The underlying theory and plans for implementation are

presented. Once developed, the brightness-temperature standard and the method for

comparison will allow microwave remote-sensing measurements to be traceable to the

primary noise standards maintained by NIST.

Keywords: brightness temperature; microwave radiometry; remote sensing; standards;

traceability
4

1. INTRODUCTION

Traceability of microwave remote-sensing measurements to primary physical

standards offers a number of benefits, all deriving from the existence of a stable, common

point of reference to which different measurements by different instruments at different

times can be compared. Thus, for example, measurements from two different satellites

could be meaningfully compared and reconciled if both were traceable to fundamental

standards. If the standards are derived from basic physical principles, then they need not

even be realized at the same laboratory. The remote-sensing instruments could be

directly traceable to the realizations of the same fundamental physical quantity at the

national measurement institutes of different countries. These different national standards

are compared periodically through formal comparisons organized by the Consultative

Committee on Electricity and Magnetism (CCEM) of the International Committee of

Weights and Measures (CIPM). At least in principle, this would allow the comparison

and reconciliation of data from U.S. and European satellites, for example. It would also

provide a stable reference point that would allow the comparison of data from satellites

flown years or decades apart, a critical issue for studying long-term phenomena such as

climate change.

Without any clarification or specification, traceability to NIST can be a weak, almost

nebulous, term in the context of remote sensing. It can refer to a single component or set

of components, such as the temperature sensors in a calibration target, or a voltmeter used

to read detector output; or it can refer to an entire instrument being calibrated against

NIST standards, as is done at infrared, optical, and ultraviolet frequencies [1]. There are

two key considerations we should bear in mind if we want to discuss traceability in a

meaningful manner: (1) What is the physical quantity whose measurement is traceable?

(2) What is the uncertainty associated with the traceability chain? These two questions

are linked, of course. In the case of microwave remote sensing, for example, the physical

quantity being measured is the received brightness temperature. The fact that the

measurement of detector voltages or temperatures within the calibration target are NISTtraceable with some small uncertainty says nothing about the measured brightness

temperature. The intervening steps from temperature within the target or detector voltage

to brightness temperature are open to question; they must be verified and the

uncertainties estimated. The more such steps there are, the larger the uncertainty

becomes. In addition, each such step and the associated uncertainty between the

traceable quantity (temperature within the target or voltage) and the ultimate measured

quantity (brightness temperature) is open to question by doubters. The more that can be

included within the traceability chain and the smaller the uncertainties, the better. This

may all be obvious to some readers, but it is worth stating clearly at the outset since it

underlies the approach and organization of this paper.

At present, NIST provides measurement services in two microwave areas that are

relevant to remote-sensing radiometry. The Thermal Noise Metrology Project measures

the noise temperature of customers’ diode (or other) noise sources. In principle, these

noise sources can then be used to measure the noise figure of a receiver or of a

radiometer without its antenna. However, such partial traceability is often of little
5

interest or use for remote-sensing radiometers. They serve a diagnostic or

characterization purpose that can often be met adequately by other means. On the other

hand, measurements of antenna pattern, offered by the Antenna Metrology Project, are

very important. We shall see below that they are required in our plan to establish a

brightness-temperature standard (as are the noise-temperature measurements), but they

are also of interest in their own right. Even if the brightness temperature were known

perfectly, it would still be necessary to know the scene to which that brightness

temperature was to be attributed, and that requires knowledge of the antenna pattern.

This paper will deal with the development of a brightness-temperature standard and the

method to transfer it, but it should be remembered that the existing antenna-pattern

measurement service constitutes a vital complement to a brightness-temperature standard.

In this paper we set out a plan to develop a link between microwave remote-sensing

measurements and fundamental noise standards maintained by NIST. Such a link can be

used to establish traceability for measurements of brightness temperature itself. NIST

does not currently maintain standards for or perform measurements of brightness

temperature at microwave frequencies. Therefore the first step in the plan must be to

develop such a capability, which would integrate aspects of the antenna and noise

measurement capabilities and standards already in place. Traceability would then be

established by comparison to NIST brightness-temperature measurements or standards, as

will be discussed later in this paper. The remainder of the paper is organized as follows.

Section 2 reviews the theory underlying our approach to linking brightness-temperature

measurements to noise standards. Section 3 outlines the plan for developing standard

radiometers as standards for microwave brightness temperature and the approximate

uncertainties that should be attainable. In Section 4 we discuss methods for comparing to

those standards, and Section 5 is devoted to the summary and conclusions.

2. THEORY FOR STANDARD RADIOMETER

We plan to develop microwave brightness-temperature standards by (reversibly)

converting the Noise Project’s present radiometers, which are used to measure coaxial or

waveguide noise sources, into standard remote-sensing radiometers that can then be used

to link microwave remote-sensing radiometer measurements to primary thermal noise

standards. This section sets out the general framework for doing so and indicates the

particular antenna properties that must be determined in order link the remote-sensing

measurements to the primary standards. For the most part, we follow the treatment in

Chapter 4 of Ulaby, Moore, and Fung [2], except that we do not rely on the RayleighJeans approximation as they do. (This difference will be discussed below.)

The basic configuration of interest is shown in fig. 1. It is similar to the usual

configuration [3] for measuring the noise temperature of a noise source except that an

antenna replaces the noise source. The output noise temperature of the antenna (at plane

x) can be measured in the same way as would the noise temperature of a noise source.

We must then relate the noise temperature measured at plane x to the brightness

temperature incident on the antenna. The brightness B(θ, φ) is the power per unit area

and solid angle incident on (or emitted from) a surface, and the spectral brightness
6

Fig. 1 Standard-radiometer configuration.

Bf(θ, φ) is the brightness per unit frequency. In terms of the spectral brightness, the

differential power received by an antenna from differential solid angle dΩ in differential

frequency interval df can be written as

dp A B F d df = eff f (θ ,φ) n (θ ,φ) Ω , (1)

where Aeff is the effective aperture area of the antenna, and Fn(θ,φ) is its normalized

antenna pattern. This assumes that the polarization of the incident radiation is matched to

the antenna. For unpolarized incident radiation the total power received in the frequency

interval ∆f is

∫ ∫

+

=

f f

f

eff f n P A B F d df

∆

π

θ φ θ φ Ω

4

( , ) ( , ) 2

1 , (2)

where the 1⁄2 arises from the average over incident polarization. In principle, of course,

Aeff and Fn are also functions of frequency, but we assume that ∆f is small enough that Aeff

and Fn are effectively constant across the range of the frequency integration. From eq.

(2) we can also identify the spectral power Pf received by the antenna ( ∫ P = P df f ),

∫ =

π

θ φ θ φ Ω

4

( , ) ( , ) 2

1 Pf Aeff Bf Fn d . (3)

To introduce the concept of brightness temperature, we consider the case of an

antenna receiving radiation from an ideal black body. The spectral brightness emitted by

an ideal black body at physical temperature T is uniform in all directions and is given by





 





− = 1

2

f 2 hf / kT e

hf B

λ , (4)

where h and k are Planck’s and Boltzmann’s constants, f is the frequency, and λ is the

wavelength. For hf/KT << 1, this assumes the familiar Rayleigh-Jeans form,

2

2

λ

kT Bf ≈ , (5)
7

where T is the physical temperature of the ideal black body.

Because of the approximate linear relationship between Bf and T for a black body,

and because the physical temperature of the scene is typically the quantity of interest in

microwave remote sensing, it is common to define and work with a “brightness

temperature” TB rather than the brightness itself. The definition most commonly used

(but which we will not use) is what can be called the equivalent black-body definition,

i.e., the brightness temperature is the temperature of an ideal black body that would give

rise to the observed brightness. Thus eq. (4) is used for any radiating body (not just an

ideal black body), with T defining the brightness temperature. With this definition, the

brightness temperature of a black body is equal to its physical temperature, but the

relationship between Bf and TB expressed by eq. (5) is valid only in the Rayleigh-Jeans

approximation. This is rather inconvenient, as will become evident below. The

alternative is to use eq. (5) to define TB,

k

B

T f

B 2

( , ) ( , )

2 λ θ φ θ φ ≡ , (6)

which we shall call the power definition. With the power definition, the correspondence

between the brightness temperature and the physical temperature for an ideal black body

is only approximate, but this definition is much more convenient for use in common

equations. In this paper, we use the power definition of TB, eq. (6). The relation between

TB and the physical temperature of an ideal black body is obtained by using eq. (4) for Bf

in eq. (6). For a non-black body, with emissivity ε less than one, a factor of ε is inserted

into the right side of eq. (4), and eq. (6) is unchanged.

Using eq. (6) in eq. (2) and assuming that ∆f is small, we have

∫ =

π

θ φ θ φ Ω

λ

∆

4

2 T ( , )F ( , )d

kA f P B n

eff . (7)

(Had we adopted the equivalent black-body definition of the noise temperature, eq. (7)

would be complicated by a Planck factor. Similarly, all the equations below in which

temperatures are added—or integrated—depend on the power definition of the brightness

temperature.) If we define the input antenna temperature TA,in by

∫ ≡

π

θ φ θ φ Ω

λ 4

, 2 T ( , )F ( , )d

A

T B n

eff

A in , (8)

then eq. (7) takes the familiar form for the available power from a passive impedance at

noise temperature TA,in,

P kT f = A,in∆ . (9)

Thus the input antenna temperature TA,in is the noise temperature (defined as the

available noise spectral power divided by Boltzmann’s constant) at the input (aperture)

plane of the antenna. Equations (8) and (9) are the sort of relationship that we need.

They give the noise power delivered to the radiometer by the antenna in terms of the

incident radiation and the antenna properties. Some work still remains to be done,

however. Equation (8) for the input antenna temperature can be put in a more useful

form by using the relationship between the (lossless) effective area and the maximum
8

directivity D0, Aeff = λ

2

D0/4π. The maximum directivity is, in turn, related to the pattern

solid angle Ωp by D0 = 4π/ Ωp, where

∫ =

π

Ω θ φ Ω

4

p Fn ( , )d . (10)

The effective area can therefore be written as

p

Aeff Ω

λ

2

= , (11)

and the input antenna temperature assumes the form

p

B n

A in

T F d

T

Ω

θ φ θ φ Ω

π

∫

= 4

,

( , ) ( , )

. (12)

Equation (12) represents the noise temperature at the antenna input plane, whereas

the radiometer measures the noise temperature at the output of the antenna, plane x in fig.

1. The two are related by (see, e.g., Ref. [4])

TA out TA in Ta (1 ) , = α , + −α , (13)

where α is the available power ratio between the antenna aperture and the output plane

(plane x in fig. 1), and Ta is the noise temperature corresponding to the physical

temperature of the antenna. Ta is given by

( ) 1

1

/ − = phys a hf kT

e

hf

k

T , (14)

where Tphys is the physical temperature of the antenna, which is assumed to be ambient

temperature. Reference [2] (and virtually all of the remote-sensing community) does not

worry about subtleties such as the distinction between available power ratio and

efficiency (delivered power ratio). They use the efficiency ηl rather than the available

power ratio α in eq. (13). They then identify ηl as the inverse of the “loss factor” L, ηl =

1/L. From our experience with adapters in noise measurements, this is a reasonable

approximation, assuming low loss and good matching. Some care will be required in

computing or measuring the loss, to ensure that the correct quantity is obtained, and

differences between α and the quantity measured or calculated will need to be accounted

for in the uncertainty analysis.

The input antenna temperature of eq. (12) integrates over the full 4π solid angle.

It therefore includes contributions from both the main beam and any side lobes, as well as

from both the target of interest and any background radiation. In common remotesensing applications, the solid-angle integration is separated into an integration over the

main lobe plus an integration over everything else. Thus the main beam defines the target

of interest. In our case, we will be viewing a calibration target, which may intercept only

a part of the main beam, or it may intercept more of the pattern than just the main lobe.

We wish to separate the calibration-target component of the brightness temperature from

everything else. We therefore separate TA,in into two parts, one containing the integration

over the solid angle subtended by the target of interest, and the other containing the rest

of the 4π solid angle. We do this for both the input antenna temperature and for the
9

pattern solid angle, eq. (10), and we define a target contribution TT and a background

contribution TBG to TA,in ,

.

( , ) ( , )

( , ) ( , )

other

targe

,

p

B n

BG

p

t

B n

T

A in T BG

T F d

T

T F d

T

T T T

Ω

θ φ θ φ Ω

Ω

θ φ θ φ Ω

∫

∫

=

=

= +

 (15)

If we further define an “antenna-target efficiency” ηAT,

p

n

AT

F d

Ω

θ φ Ω

η

∫ ≡ target

( , )

, (16)

we can write

TA in AT TT AT TBG (1 ) , =η + −η , (17)

where

. ( , )

( , ) ( , )

( , )

( , ) ( , )

other

other

target

target

∫

∫

∫

∫

=

=

θ φ Ω

θ φ θ φ Ω

θ φ Ω

θ φ θ φ Ω

F d

T F d

T

F d

T F d

T

n

B n

BG

n

B n

T

 (18)

In eq. (17), TT is the incident brightness temperature averaged over the portion of the

antenna pattern corresponding to the target solid angle, which is the quantity of interest

for the standard radiometer. The antenna-target efficiency ηAT is the fraction of the full

antenna pattern that is subtended by the target of interest. The quantity TBG is the

average brightness temperature from directions other than the target.

We can use eq. (17) in eq. (13) to obtain an equation for TT in terms of measured

quantities,

( ) ( ) a

AT

BG

AT

AT

A out

AT

TT T T T

αη

α

η

η

αη

− − − = − 1 1 1 , . (19)

To control the effect of TBG in eq. (7), we need to control the environment in which the

standard radiometer operates. We intend to use an enclosure with absorptive walls,
10

maintained at room temperature, which will also be the temperature of the antenna, Ta. In

that case, TBG = Ta , and eq. (7) becomes

( ) A out a

AT

TT = Ta + T , − T 1

αη

. (20)

Equation (20) is the desired result for our standard-radiometer measurements. It allows

us to determine TT , the average incident brightness temperature received from the target,

in terms of TA,out, α, ηAT, and Ta. In eq. (20) TA,out is the noise temperature at the output of

the antenna, measured by the radiometer; α is the available power ratio between the

antenna aperture and its output, approximately equal to 1/L; ηAT is the antenna-target

efficiency, defined in eq. (16) and determined from the normalized antenna pattern; and

Ta is the noise temperature corresponding to the physical temperature of the antenna and

the enclosure.

 

3. PLAN FOR STANDARD RADIOMETER

If eq. (20) is to be used to characterize a radiometer, the two antenna properties

we need are α, which is the available power ratio between the antenna aperture and its

output, approximately equal to the inverse of the loss factor 1/L, and ηAT, which is the

antenna-target efficiency. Ohmic losses are best determined by calculation, using any of

a number of software packages. For common, commercial standard-gain horns the ohmic

losses are found to be less than 0.03 dB with the worst-case conductivity (at about 20

GHz). However, the uncertainty in the electrical conductivity is the major uncertainty in

determining the ohmic losses. The normal practice is to neglect the ohmic loss and

include an additional uncertainty of 0.03 dB to account for its neglect [5]. A possible

alternative method would be to measure the gain and the directivity of the antenna by two

independent methods and to obtain the loss from the ratio of gain to directivity.

However, a preliminary estimate indicates that it would have uncertainties comparable to

those in the calculational method. (It could still be useful as a check.)

The antenna-target efficiency ηAT is defined in eq. (16). To determine it, we must

measure the antenna pattern over all angles for which the normalized antenna pattern

Fn(θ,φ) is not negligible. NIST pioneered the use of near-field scanning for antennapattern measurements [6] and continues to maintain and use several near-field ranges for

this purpose. The frequency range covered is currently 2 GHz to 75 GHz, but the upper

limit will be extended to 110 GHz in the very near future. The antenna group is currently

conducting an experiment to determine how well they can measure the pattern of a

standard-gain horn. These measurements will allow us to estimate the uncertainty with

which ηAT can be determined.

From the standpoint of noise measurements, the framework developed in Section

2 requires only a measurement of the noise temperature at the input plane to the receiver

section of the radiometer. This corresponds to the input plane for the coaxial and

waveguide radiometers that NIST has built and with which NIST has been measuring the

noise temperature of noise sources for several decades. Current capability includes two

discrete points at low frequency (30 MHz and 60 MHz) and continuous coverage from 1
11

GHz to 65 GHz. Cryogenic coaxial standards [7] are used up to 12.4 GHz, and cryogenic

waveguide-horn standards [8] are used above 12.4 GHz. The typical uncertainty in the

noise temperature measured at the input plane depends on the frequency and the noise

temperature being measured. For the typical 10 000 K diode noise source that NIST

measures, the standard (1-sigma) uncertainty is about 0.4 % or 0.5 % for frequencies up

to about 20 or 25 GHz, and it increases to around 0.9 % at 65 GHz. For noise

temperatures in the 200 K to 300 K range, however, the uncertainties are considerably

smaller, approximately 0.3 K at 20 or 25 GHz.

Because we do not normally measure noise temperatures in the 200 K to 300 K

range, we thought it prudent to test whether our uncertainty estimates were reliable. To

do so, we designed and constructed a variable noise source whose noise temperature is

known and can be varied in a range around ambient temperature (296 K). The noise

temperature was then measured, and the measured and predicted noise temperatures were

compared. This was done on our coaxial radiometer for frequencies from 2 GHz to 12

GHz, for noise temperatures from 263 K to about 320 K, with very good results [9]. We

conclude that our uncertainty estimates are approximately correct (at least for this

radiometer) and that we can measure noise temperatures around 250 K to 300 K with a

standard uncertainty of about 0.3 K.

The final component in using eq. (20) as the basis for a standard radiometer is to

control the environment, so that TBG = Ta . For this purpose, we plan to use the standard

radiometer in a chamber that would exclude outside radiation and present a uniform

background radiation at Ta = 296 K. A preliminary sketch of such a chamber is shown in

fig. 2. It is similar to a radio-frequency (RF) anechoic chamber, with metal (probably

aluminum) exterior walls, lined on the inside with RF absorber. A major difference from

an RF anechoic chamber is the temperature control that is required. We expect to house

the chamber within a temperature-controlled room to maintain the outer walls at the

desired temperature. Fans will be used to circulate the air, preventing the development of

hot or cold spots, and an array of thermistors will be embedded in the walls to monitor

the temperature. A positioning table or assembly will be located near the back wall,

suitably shielded with absorber to obscure the electronics behind it. The required

precision and sophistication (and concomitant cost) of the positioner is not clear at this

time. The size of the chamber will require some study. We want the target to be in the

far field of the antenna, and we want the chamber to be usable down to about 18 GHz. If

the common antenna far-field criterion of 2d2

/λ is used, the chamber would need to be

only about a quarter meter long, which is conveniently small. However, for some

purposes [10], the far-field criterion for a standard-gain horn is about 32(d2

/λ), which

would require a chamber length of about 4 m. Clearly, this issue must be addressed

before chamber design and cost estimates can be done. The NIST waveguide radiometers

and cryogenic standards are already kept on carts and can be rolled into position at the

front of the chamber. The size of the opening at the front of the chamber will be

determined by a compromise between shielding and air circulation. If necessary, it

would be simple to design shielded air vents for placement in the walls.
12

Figure 2. Preliminary sketch of chamber for standard radiometer.

The principal disadvantage of such a chamber is the lack of a capability for

thermal-vacuum (TV) testing. Space-based radiometers operate in a vacuum, and

although the temperature is controlled, it is not likely to be 296 K. Since the heat transfer

mechanisms depend on the surrounding atmosphere, the thermal properties of a

calibration target will be different in a TV chamber from those in the proposed “ambient”

chamber. It is therefore desireable to test under the anticipated operating conditions, i.e.,

in a TV chamber. At this time, however, such a facility appears to be beyond the

anticipated resources of NIST. The next section includes a suggestion for how we can

compensate for the lack of a TV environment for the proposed brightness-temperature

standard. There are also possible ways to mitigate the effects. For example, one

significant factor in the possible difference between behavior in a TV chamber and in an

ambient chamber is that in a TV chamber there is virtually no convective heat transfer,

whereas in an ambient chamber there is. The effect of this difference can be minimized

by fitting a styrofoam cover over the front surface of the target, to eliminate its thermal

contact with the surrounding atmosphere.

To estimate the uncertainty achievable with a standard radiometer, we refer to eq.

(20). We expect to be able to control Ta to within about 0.2 K. As we noted above, TA,out

can be measured with a standard uncertainty of about 0.3 K up to about 26 GHz. The

Temperature-monitoring

Electronics

Waveguide

Radiometer

Cryogenic

 Noise

Standard

Ambient

 Noise

Standard

Horn

Target

Mounting

Plate

Temperature Control

And Alignment

Systems

Temperature Sensors

Absorber

Aluminum Housing

Air circulator

Air Circulator

Waveguide Switch
13

uncertainty in α for a standard-gain horn will be about 0.5 %. We do not yet know the

uncertainty in ηAT. If it can be kept to about the same as that in α, we should be able to

achieve a standard uncertainty in TT of about 0.3 K to 0.8 K for TT between 200 K and

300 K, at frequencies up to about 26 GHz.

4. BRIGHTNESS-TEMPERATURE STANDARD AND TRANSFER

The standard radiometer(s) discussed above will constitute a standard for

brightness temperature, linking measurements of it to primary noise standards. For it to

be useful, however, there must be a means for others to compare to this standard. One

way would be for outside parties to send their calibration targets to NIST, where the

targets could be measured by the standard radiometers in the NIST ambient chamber. It

would also be very useful for NIST to have a transportable transfer standard, which could

be measured by the customer’s radiometer at the customer’s facility. For this purpose,

we plan to develop a standard calibration target, in addition to the standard radiometers.

Its temperature will be monitored by several thermistors, so that it will constitute a

brightness-temperature standard in its own right, but it also will be measured by the

standard radiometers, thereby providing a check of both the standard target and the

radiometers. This standard calibration target will be designed for use in TV chambers, so

that it can be used in the same facilities that are used for pre-launch testing of a

customer’s radiometer and its calibration targets. An integral part of the development

and design of the NIST standard target will be the development of methods for testing

and characterizing such targets. In particular, we will attempt to develop test methods for

detecting thermal gradients within the target (infrared imaging) and for measuring

reflectivity or emissivity of the target.

Assuming both standard radiometers and a transportable standard target are

developed, there will be a number of strategies available for establishing traceability

through comparison to a NIST brightness-temperature standard. In the NIST chamber, a

customer’s calibration target could be measured directly with a NIST standard

radiometer, thereby linking its brightness temperature to primary noise standards (under

ambient conditions). A customer’s radiometer (within size limitations) could also be

measured in the NIST chamber and compared directly to the NIST standard radiometer

by measuring the same target with both radiometers. The standard target could also be

measured at a customer’s facility, either under ambient conditions or in a TV chamber. A

comparison between the performance of the NIST standard target in the NIST ambient

chamber and in a TV chamber could be done if a portable radiometer was available. The

standard target could then be measured by the same radiometer in both the ambient

chamber and in a TV chamber. It would also be measured with the NIST standard

radiometer in the ambient chamber. This would provide a link of the portable radiometer

to the NIST standard radiometer, an opportunity that could be attractive enough to induce

the owner of a portable radiometer to participate in such a comparison.
14

5. SUMMARY

We plan to develop standards for microwave brightness temperature for the

frequency range 18 to 65 GHz, starting with the WR-42 (18 to 26.5 GHz) band. This will

be done by connecting characterized antennas to the measurement ports of existing NIST

waveguide radiometers, which are calibrated against primary noise standards. Thus the

brightness temperature standard will be tied to the primary noise standards. A chamber

will be constructed to provide a controlled (but not TV) environment for using these

radiometers. At the lowest frequencies, we anticipate a standard uncertainty of about 0.3

K to 0.8 K in the brightness temperature for temperatures in the 200 K to 300 K range. In

parallel, we plan to develop a portable standard calibration target, which will be suitable

for use in a TV environment. The portable standard calibration target will be crosschecked with the standard radiometers. The standards will enable the measurement of

customers’ radiometers or targets at NIST at ambient temperature and pressure or

measurement of the NIST standard target at a customer’s facility under either ambient or

thermal-vacuum conditions.

__________________

For helpful comments, information, and discussions, I am grateful to other

members of the NIST Radiometer Calibration Project, including A. Cox, G. Free, D.

Walker, M. Francis, K. MacReynolds, J. Guerrieri, and R. Wittman. I am also grateful to

E. Gerecht and C. Holloway for suggestions on the original manuscript. This work was

supported in part by the NOAA/NESDIS/Integrated Program Office.

6. REFERENCES

[1] J.P. Rice and B.C. Johnson, “NIST activities in support of space-based

radiometric remote sensing,” Proc. SPIE, vol. 4450, pp. 108 – 126, 2001.

[2] F. Ulaby, R. Moore, and A. Fung, Microwave Remote Sensing: Fundamentals and

Radiometry, Vol. 1, Artech House, Norwood, MA, 1981.

[3] J. Randa and L.A. Terrell, “Noise-temperature measurement system for the WR28 band,” NIST Tech. Note 1395, August 1997.

[4] G. Engen, Microwave Circuit Theory and Foundations of Microwave Metrology,

Ch. 20, Peter Peregrinus, London, 1992.

[5] J. Lemanczyk, F.H. Larsen, J.E. Hansen, and J. Aasted, “Calibration of standard

gain antennas using a spherical near-field technique,” IEE Conference on

Antennas and Propagation, York, England, April 1981.
15

[6] R.C. Baird, A.C. Newell, and C.F. Stubenrauch, “A brief history of near-field

measurements of antennas at the National Bureau of Standards,” IEEE Trans.

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